DESCRIPTION
The invention relates to a diode-addressed color display comprising an UV-diode and a phosphor for luminous representations, luminaries, solid state image intensifiers, display screens and monitors, and the like.
Color displays for luminous representations, luminaries, solid state image intensifiers, display screens and monitors should reproduce colored images in a color-sensitive manner. To achieve this, all color information about a colored image is represented by information about the three primary colors red, green and blue. By additive color mixing, any color, including white can be made from said three primary colors. This principle is employed by conventional color television receivers comprising a Braun tube as well as by the various flat-tube technologies, such as plasma display screen, electroluminescent display screen and LCD-displays. In other commercially available color displays, the color triad red, green and blue is generated by diode arrays comprising red, green and blue-emitting semiconductor diodes. In these color displays, however, the color-sensitive image reproduction, particularly the color-pure reproduction of green and blue is problematic. By virtue of the development of UV-emitting semiconductor diodes, the possibilities of a color-sensitive image reproduction for diode-addressed color displays has been increased, because, theoretically, any desired color of the visible light spectrum can be generated from UV-light. For this purpose, use is made of phosphors which absorb UV-light and reradiate it with a wavelength in the visible region. It is known that for this conversion of the UV-light into the visible region, use is made of inorganic pigments as phosphors. For example, it is known from Jpn. J. Appl. Phys. Vol. 35 (1996) pp. L838-839 to use ZnS:Ag, ZnS:Cu,Al and ZnCdS:Ag as the phosphors for converting the UV-light from UV-diodes in a fluorescent color display. These and other customary phosphors, such as Y.sub.2 O.sub.3 :Eu have a high quantum yield, but their absorption in the near-UV region, in which the UV-diodes emit, is very small.
Therefore, it is an object of the invention to provide a diode-addressed color display comprising an UV-diode and a phosphor, which phosphor has a high quantum efficiency and a high extinction coefficient in the near-UV wavelength range and enables a color-sensitive image reproduction to be achieved.
In accordance with the invention, this object is achieved by a diode-addressed color display comprising an UV-diode and a phosphor of the general formula [Eu(diketonate).sub.a X.sub.b1 X'.sub.b2 ], wherein X=pyridine or a unidentate pyridine derivative and X' =2,2'-bipyridine or a 2,2'-bipyridine derivative, and 2a+b.sub.1 +2b.sub.2 =8. Such a color display is distinguished by a high intrinsic emission quantum efficiency and a ligand-centered absorption in the range between 350 and 400 nm with a high extinction coefficient. After photophysical considerations, these two phosphor properties are in fact mutually exclusive. Surprisingly, it has been found, however, that phosphors of the general formula [Eu(diketonate).sub.a X.sub.b1 X'.sub.b2 ] meet both criteria. The absorbing capacity of the inventive phosphors comprising the "antenna molecules" X, X' is a multiple of that of classic phosphors. A variation of the ligands X and X' enables the substantially linear, independent introduction of high absorptions at different wavelengths in the europium compounds. Concentration quenching, which is a general problem with classic phosphors having a high activator concentration, is not observed in the phosphors in accordance with the invention. The phosphors in accordance with the invention are molecular compounds and hence, in general, readily soluble in polar organic solvents. As a result, their properties can be readily examined in solution and the results can be transferred to the solid state. The solubility in organic solvents additionally enables new design concepts for diode-addressed color displays to be developed.
Within the scope of the invention, it is preferred that a=3. It may alternatively be preferred that a=2. These europium complexes are charged and can be bound to a polymer carrier, such as Nafion.RTM.. As a result, these phosphors are very stable and the photophysical properties further improved.
It is further preferred that the diode-addressed color display comprises a transparent polymeric coating which contains the phosphor of the general formula [Eu(diketonate).sub.a X.sub.b1 X'.sub.b2 ] in solid solution. The coating is transparent because the light is not, scattered at the dissolved phosphor particles.